Atrial fibrillation in horses part 1: Pathophysiology.

This research article reviews the pathophysiology of atrial fibrillation (AF), the most common clinical cardiac irregularity observed in horses. The research also investigates certain predisposing factors and alterations that could instigate AF, such as structural heart disease, and gives further insight into how the condition evolves and influences the cardiac function of a horse.

Understanding Atrial Fibrillation

• Atrial fibrillation is a heart disorder affecting the atrium’s rhythm. It is the most recurrent clinical arrhythmia seen in horses, with rates reaching as high as 2.5%.
• This condition stems from a combination of electrical triggers and an exposed substrate. These triggers can take the form of premature depolarizations in the atria due to changes in automaticity or triggered activity, or localized microreentry.
• The progression of the disorder is influenced by alterations in atrial myocardial ion channels, Ca handling mechanisms, structural anomalies, and imbalances in the autonomic nervous system.

Underlying Predisposing Factors

• Structural heart disease is one of the major predisposing factors leading to AF. Conditions like valvular regurgitation result in persistent atrial stretch which can instigate AF.
• ‘Lone AF’ or idiopathic AF is a form of the disorder where no clear underlying cardiac issue is detected using standard diagnostic procedures. Horses with this condition may suffer from undiagnosed microstructural alterations in the heart’s muscle or ion channel dysfunctions.

Atrial Fibrillation Remodelling Impact

• The occurrence of atrial fibrillation leads to electrical, contractile, and structural remodelling. This remodelling process furthers the maintenance of AF within the heart.
• Electrical remodelling results in a shortening of the atrial effective refractory period, promoting reentry. Contractile remodelling reduces myocardial contractility, while structural remodelling leads to interstitial fibrosis and atrial enlargement.
• Reverse remodelling occurs post cardioversion to sinus rhythm, but complete recovery could take weeks or even months depending on the duration of AF.

Clinical Symptoms of Atrial Fibrillation

• The clinical signs associated with atrial fibrillation are contingent on the aerobic demands during exercise, the ventricular rhythm response, and the presence of underlying cardiac anomalies.
• Horses with ‘lone AF’ generally don’t exhibit clinical symptoms at rest. However, during exercise, they may display poor performance, exercise-induced pulmonary hemorrhage, respiratory distress, weakness or in rare cases, may collapse.